Re: [RFC PATCH v9 for 4.15 01/14] Restartable sequences system call

[Andy Lutomirski <luto@amacapital.net>]

On Fri, Oct 13, 2017 at 7:27 AM, Mathieu Desnoyers
<mathieu.desnoyers@efficios.com> wrote:
> ----- On Oct 13, 2017, at 9:56 AM, Florian Weimer fweimer@redhat.com wrote:
>
>> On 10/13/2017 03:40 PM, Mathieu Desnoyers wrote:
>>> The proposed ABI does not require to store any function pointer. For a given
>>> rseq_finish() critical section, pointers to specific instructions (within a
>>> function) are emitted at link-time into a struct rseq_cs:
>>>
>>> struct rseq_cs {
>>>          RSEQ_FIELD_u32_u64(start_ip);
>>>          RSEQ_FIELD_u32_u64(post_commit_ip);
>>>          RSEQ_FIELD_u32_u64(abort_ip);
>>>          uint32_t flags;
>>> } __attribute__((aligned(4 * sizeof(uint64_t))));
>>>
>>> Then, at runtime, the fast-path stores the address of that struct rseq_cs
>>> into the TLS struct rseq "rseq_cs" field.
>>>
>>> So all we store at runtime is a pointer to data, not a pointer to functions.
>>>
>>> But you seem to hint that having a pointer to data containing pointers to code
>>> may still be making it easier for exploit writers. Can you elaborate on the
>>> scenario ?
>>
>> I'm concerned that the exploit writer writes a totally made up struct
>> rseq_cs object into writable memory, along with function pointers, and
>> puts the address of that in to the rseq_cs field.
>>
>> This would be comparable to how C++ vtable pointers are targeted
>> (including those in the glibc libio implementation of stdio streams).
>>
>> Does this answer your questions?
>
> Yes, it does. How about we add a "canary" field to the TLS struct rseq, e.g.:
>
> struct rseq {
>         union rseq_cpu_event u;
>         RSEQ_FIELD_u32_u64(rseq_cs);  -> pointer to struct rseq_cs
>         uint32_t flags;
>         uint32_t canary;   -> 32 low bits of rseq_cs ^ canary_mask
> };
>
> We could then add a "uint32_t canary_mask" argument to sys_rseq, e.g.:
>
> SYSCALL_DEFINE3(rseq, struct rseq __user *, rseq, uint32_t, canary_mask, int, flags);
>
> So a thread which does not care about hardening would simply register its
> struct rseq TLS with a canary mask of "0". Nothing changes on the fast-path.
>
> A thread belonging to a process that cares about hardening could use a random
> value as canary, and pass it as canary_mask argument to the syscall. The
> fast-path could then set the struct rseq "canary" value to
> (32-low-bits of rseq_cs) ^ canary_mask just surrounding the critical section,
> and set it back to 0 afterward.
>
> In the kernel, whenever the rseq_cs pointer would be loaded, its 32 low bits
> would be checked to match (canary ^ canary_mask). If it differs, then the
> kernel kills the process with SIGSEGV.
>
> Would that take care of your concern ?
>

I would propose a slightly different solution: have the kernel verify
that it jumps to a code sequence that occurs just after some
highly-unlikely magic bytes in the text *and* that those bytes have
some signature that matches a signature in the struct rseq that's
passed in.